A frequency decomposition of normal speech will indicate components higher than 4000 Hz; however, the total energy in these components is substantially smaller than that found in those frequency components below 4000 Hz. For this reason it is common to consider, as will be done here, an analog channel with transmission bandwidth from 0-4000 to be unrestricted. The most common cause of bandwidth restriction is the application of filtering stages by analog transmission channel providers. This filtering is normally performed in the channel equipment before multiplexing channels onto an analog carrier using frequency division multiplexing techniques, or onto a digital carrier using time division multiplexing techniques. The result is frequency dependent attenuation in the high and low fringes of the 0-4000 Hz band with the positioning and steepness of the resulting channel roll-off behavior dependent on the channel provider's selection of filtering equipment and the number of multiplexing stages over the channel. A channel impairment that is a natural consequence of this filtering process is frequency dependent group delay, which causes differential delay distortion of the data signal, and which is most strongly exhibited in the filter roll-off frequency region. Another impairment common to analog channels is the induced power line and telephone signalling noise often present in the 0-300 Hz frequency band. The effect of analog channel impairment and bandwidth restriction has been to limit the data throughput performance of speech and data multiplexors of the prior art as is now discussed.
Existing means for the simultaneous transmission of speech and data over analog channels fall into several classes.
One class employs the silent intervals in speech for the data transmission, while another uses the speech itself as a carrier onto which data is modulated. Another approach is to superimpose the data signal on the voice signal, with the separation of each component from the transmission distorted composite signal performed by the receiver. Yet another approach is to distribute the data energy in a spread spectrum fashion that would prove inaudible to the listener.
The class of methods within which the present invention falls generally is the class which relates to the division in frequency of the available bandwidth, with speech and data signals assigned to separate frequency sub-bands.
In the prior art in this class, when the analog transmission channel is the local loop portion of the public telephone network, methods for data transmission well above the voice frequency band have been disclosed as in U.S. Pat. No. 4,302,629 issued to J. D. Foulkes et al. on Nov. 24, 1981. Such schemes require bandwidths well in excess of the 0-4000 Hz frequency range and hence these implementations are unsatisfactory over bandwidth restricted channels as presented by what are termed loaded local loops. These loops, which are generally over 18,000 feet in length, are circuits in which inductors, called loading coils, have been installed to improve frequency response in the 2200-3200 Hz range. Such loops severly attenuate frequencies above approximately 3700 Hz (Transmissions Systems for Communications, Bell Telephone Laboratories pp. 224-226) and comprise approximately 20 percent of all local loops in the United States public telephone network.
Another approach in the prior art is to employ the frequencies between 0 and 200 Hz and a single carrier modulation scheme to produce a 200-300 bps data transmission rate. A disadvantage of this method is that single carrier transmission is sensitive to the composite noise over the entire sub-band employed, and in this case power equipment induced 60 Hz and 180 Hz noise (Bell System Technical Journal, Vol. 63, No. 5, pp. 775-818) will cause signal distortion. This distortion limits the information carrying ability of the carrier and results in a relatively low data rate. A further disadvantage is that frequencies below 200 Hz are either unavailable or severely attenuated on long distance public switched telephone network links (BSTJ Vol. 63, No. 5, pp. 775-818).
A similar approach in the prior art is disclosed in U.S. Pat. No 4,011,407 issued to N. DiSanti and F. Oster on Feb. 26, 1976, in which a single data carrier is employed in a 100 Hz band centered at 2900 Hz. The data rate achieved in this case is 250 bps with indications that 500 bps is possible with a doubling of the data bandwidth to 200 Hz. This low data rate is a consequence of the inability to utilize a broader sub-band with a single carrier modulation scheme, since such a broader band would be subject to frequency dependent attenuation and group delay (BSTJ Vol. 63, No. 9, pp. 2059-2119), and single carrier modulation is sensitive to such impairments.
A third approach in the prior art is that disclosed in U.S. Pat. No. 4,546,212, issued to J. Crowder, Sr., on Oct. 8, 1985, for use over the public telephone network in which two sub-bands are proposed, 0-800 Hz for speech, and 800 Hz and above for data. The disadvantages of this scheme are first, that the speech quality is very poor due to the severely limited bandwidth allowed for voice transmission, and second, that a switching means must be employed to allow full bandwidth access in order that DTMF (dual tone multi-frequency) call signalling tones (which spans the 800 Hz limits) will be possible.